Study of soot production for double injections of n-dodecane in CI engine-like conditions

Abstract

Soot production mechanism in multiple injections is complex since it involves its dependence on turbulent interactions of constituting injections and their combustion progress. A concise study was performed in a constant-volume combustion vessel by considering a double injection scheme of 0.3 ms pilot injection, 0.5 ms dwell time and 1.2 ms main injection (nomenclature: 0.3/0.5/1.2 ms) with n-dodecane as fuel and replicating the thermodynamic operating condition of a compression ignition (CI) engine. Experimental ambient temperature variations of 900 K and 800 K were performed at 15% ambient oxygen level. Simultaneous planar laser-induced fluorescence (PLIF) of formaldehyde and schlieren imaging techniques were employed to analyze the ignition and flame characteristics experimentally. These studies revealed almost similar heat release rates for a double injection at 900 K and 800 K ambient gas temperatures due to combustion of a longer main injection which is enhanced by pilot combustion event. A lower soot production for 800 K ambient condition over 900 K case was observed, which was concluded to be due to its higher lift-off length which would allow for a leaner combustion of fuel-air mixtures. Numerical simulations were performed using a Large Eddy Simulation (LES) approach by extensively validating the 900 K double injection condition with respect to non-reacting vapor penetration profiles of both injections, reacting jet heat release rate and spatial as well as temporal (qualitative) soot production. As part of LES work, a dwell time variation of 0.65 ms (0.3/0.65/1.2 ms) was performed to reveal the sensitivity of soot production to variations in dwell time. It was observed numerically that marginally higher quasi-steady lift-off length of the 0.3/0.65/1.2 ms injection causes increased entrainment of surrounding oxygen into the flame region. This leads to combustion of slightly leaner fuel-air mixture and hence relatively less soot when compared to a 0.3/0.5/1.2 ms injection.

Optical investigations on Diesel spray dynamics and in-flame soot formation

“…From Figure 5.26 one can conclude that the soot mass formed by the second injection pulser is larter with a shorter dwell time, which is consistent with the CFD results from [23]. As shown in the inert spray analysis, the equivalence ratio and momentum flux distribution of the spray during the initial stages of the second injection is not dependent on dwell.…”

“…The effects of ambient conditions and end-of-injection transients on combustion have been investigated by Genzale et al [21][22][23]136]. They observed that the likelihood of combustion recession decreases with lower ambient temperature and oxygen concentration.…”

“…As a consequence, the sum of soot mass (m soot ) along the line-of-sight at each 60 3. Tools and Methodology pixel was derived from Eq.3.7 using an assumed density of 1.8 g{cm 3 for soot (ρ soot ) [23]. .7 shows one example of the soot optical extinction (KL) distribution calculated from Eq.3.6.…”

“…Combustion recession under relatively lean conditions can help to reduce unburned hydrocarbons, but if mixtures are too rich, combustion recession can promote increased soot emissions. To link end-of-injection transients to ambient thermodynamic conditions and injector parameters, a scaling methodology for the likelihood of combustion recession in diesel sprays was also developed by Genzale et al [23].…”

“…With increased entrainment of ambient gas with low axial momentum, vapor-fuel mixtures near the injector transitioned from fuel-rich to fuel-lean immediately after the end of injection (EOI) and the entrainment wave decelerates the flow and causes it to stagnate near the injector, which could contribute to incomplete combustion, unburned hydrocarbon (UHC) or combustion recession [18][19][20][21][22]. The most influential parameters for combustion recession are the ambient thermodynamic conditions, injector parameters, and the EOI transient [23]. Apparently, it is not able to be observed from this case, as shown in Figure 2.2 at 1720 µs ASOI.…”